O negative blood can be given to anybody it is called the universal donor. It’s not extremely rare. Approx between 15 and 20 % of population are O negative. It is the blood group that is kept on standby (we always have 2 fresh units in a fridge next to the operating theatre) in case of a major emergency, if there is not time to crossmatch and give group specific blood. So if you are group O negative and are old enough to donate the blood banks Really need your blood. If you are AB positive your probably not so popular in the blood banks as your blood can only be given to other AB positive patients and they are quite rare. And just to complicated things AB POSITIVE patients can get any blood group. Hope this helps!